In image forming devices such as an electrophotographic device, an electrostatic recording device, and an electrostatic printing device, an image formation method which forms a desired image by developing an electrostatic latent image to be formed on a photo conductor with a toner for developing electrostatic images is widely performed. The method is applied to a copier, a printer, a facsimile and their composite machines thereof.
For example, in the electrophotographic device using electrophotography, in general, the surface of a photoconductor consisting of a photoconductive substance is uniformly charged by various means, and then an electrostatic latent image is formed on the photoconductor. Subsequently, the electrostatic latent image is developed using a toner, and the toner image is transferred onto a recording material such as a sheet of paper, and the material is heated so as to fix the image, whereby a copy is obtained.
As for the toner used in an image forming device, in order to improve functions such as the charge stability and fluidity of toner, external additives, such as inorganic particles and organic particles having a particle diameter lower than that of colored resin particles (toner particles) are generally attached and added onto the surface of toner particles (external addition) before use.
In the toner obtained using the conventional external additive, it is sometimes hard to obtain a charge rising property of toner at an early stage of printing. Particularly, the charge rising property of toner is easily influenced by the usage environment in a severe environment such as a low temperature and low humidity environment or a high temperature and high humidity environment. As a result, the image quality is easily deteriorated by fogging at the early stage, and an adverse effect is given to early-stage printing performance.
In the continuous printing process of a large number of sheets, mechanical stress in a development device (the number of times of contact of toner particles with each other is increased by stirring) causes tendency of burying of an external additive to the toner particle surfaces and/or defects of being liberated (detached) from the toner particle surfaces. It becomes difficult to impart a stable charging ability (charge stability) to the toner particles over time. Thus, there is a need for development of a toner in which the above problems are solved.
Patent Literature 1 discloses an electrophotographic developer comprising a toner containing at least a binder resin, a colorant, a release agent and inorganic particles, wherein titania (inorganic particles) is contained as an external additive, and there is a peak in an absolute value of charge amount of 20 to 40 μC/g, and a total of the distribution function values (14 μC/g or less) is 0.8 mg/10 g in a distribution correlation diagram of the charge amount which is measured by a blowoff method.
Incidentally, a toner is charged by contact with charge members such as a carrier and a blade, and the charge amount of each of the toner particles varies. The variations in toner charge amount causes variations in development property. Further, a toner with little charge amount and a toner oppositely charged are developed to a non-image area and cause not only fogging and waste of toner but also toner scattering, and further produce detects such as image soiling and contamination of inside of the development device. In order to resolve such problems, it is preferable that the toner charge amount less varies. Also, a toner whose charge amount is specified is suggested.
For example, Patent Literature 2 discloses a developing method that uses a non-magnetic one-component toner containing at least a binder resin, a colorant and a charge control agent and having a surface coated with an external additive, wherein a peak of (Q/D) in distribution (Q/D) which is expressed by charge amount (Q) and toner particle diameter (D), the percentage of the number of particles within a specific range, and the percentage of the number of oppositely charged particles are adjusted to a specific range. Patent Literature 3 discloses a developing method comprising the steps of: supplying a toner comprising base particles and an external additive from a toner supply unit to a developer stirring unit; mixing the supplied toner with a developer by an stirring member; charging the toner to be carried and feed onto a developer carrying support; and visualizing an electrostatic latent image formed on an latent image carrying support, wherein the amount of weakly and/or oppositely charged toner components in the charged toner amount distribution at an inlet of the developer stirring unit immediately after supplying the toner, and the amount of weakly and/or oppositely charged toner components in the charged toner amount distribution at an outlet of the developer stirring unit immediately before the toner passes through the developer stirring unit and is carried onto the developer carrying support are satisfy a specific relationship.
Patent Literature 4 discloses a toner comprising at least a binder resin, a colorant and a wax, wherein when the charged particle amount distribution is measured by the laser Doppler method, the proportion of the particles having a dwell-time of 3.0 to 10.0 ms is in a range of 40 to 90% based on all the toner particles, an absolute value of a peak top of main peaks in distribution (q/d) which is expressed by charge amount (q) and toner particle diameter is within a specific range, and the half width of the main peaks in distribution (q/d) is in a specific range.
However, Patent Literatures 2 to 4 disclose that the charge amount in the toner particles varies and the use of those toners allows for output of a stable image. The present inventors have examined and found that if the toners are superimposed and printed, the charging ability of the toners needs to be further controlled.